| Summary: | © 2014 Dr Sarah Fiona Barker Water scarcity is of increasing concern in many areas of the world and is a powerful driver for the expansion of wastewater reuse, particularly in arid regions. Agricultural irrigation is the predominant use of wastewater, although recently there has been interest in treated wastewater for supplementation of drinking water supplies. The increasing use of wastewater highlights the need for risk assessment to inform evidence-based guidance to minimise both human health and environmental impacts. The research focus of this thesis was restricted to human health risks. Quantitative Microbial Risk Assessment (QMRA), a probabilistic modelling technique, was used to evaluate the risks associated with wastewater reuse and to determine the required pathogen reductions for safe reuse. However, risk assessment is often constrained by a paucity of model input data and information on pathogen concentrations is seldom available. While simplistic methods have been widely used to overcome this impediment, a key objective of this work was to explore more rigorous methods to estimate pathogen concentrations. Four pertinent water reuse scenarios were used to explore and challenge existing QMRA methods. These scenarios considered consumption of vegetables irrigated with wastewater in Australia and Ghana, and potable reuse in Antarctica. The scenarios assessed different water qualities, from highly-treated municipal wastewater to untreated domestic greywater and drainage water, and covered different scales of reuse including large commercial systems and small, backyard reuse. This work contributed to the body of knowledge relating to levels of risk for wastewater reuse scenarios. In particular, epidemiological methods were developed to estimate pathogen concentrations as an alternative to the widely-used ratio method. Broader population considerations were identified as particularly important in assessing risk. Person-to-person interactions affect disease transmission dynamics, wastewater quality, treatment requirements and potential health risks from wastewater reuse. As well, population-level risk considers both exposed and unexposed individuals, which is important particularly when using currently-accepted health targets. The broader context of the risk scenario should also be considered, including total risk (from all key pathogens), community prevalence of disease, and the contribution of other exposure pathways, and further consideration is required to assess the appropriateness of health targets and how they are applied. Overall, risk estimates were low for consumption of raw vegetables irrigated with highly treated wastewater or with greywater for single-dwelling reuse. Risks to salad consumers in Ghana were significantly higher and exceeded the 10-4 Disability Adjusted Life Years (DALY) health target. As well, the need for additional treatment barriers was suggested for direct potable reuse in a small remote community due to the dynamics of infection and illness in this population. Interventions such as produce washing and withholding periods were found to significantly reduce estimates of disease burden and an assessment of model structure suggested that using wastewater quality to estimate food consumption risks may result in an underestimate of risk. This research demonstrated that wastewater reuse can be safely expanded as a means to address current and future water shortages; however, in most cases this is contingent upon safe and reliable water treatment systems and/or interventions.
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